This application is a 371 of PCT/US90/06662, filed Nov. 14, 1990.
1. Field of the Invention
This invention relates to a process for recovery of palladium from aqueous acidic solutions containing palladium by a solvent extraction process and to reagents employed in the process.
2. Statement of Related Art
During the recovery of precious metals from ore or scrap including spent catalysts, the use of solvent extraction to separate the precious metals from one another and from base metals that may also be present is becoming more widespread. The hydrometallurgical processes employed for the separation and recovery of the platinum group metals (PGMs), e.g. platinum, palladium and rhodium, typically involve dissolving the metal values by some type of oxidative acidic chloride leach, typically with aqua regia or HCl/Cl.sub.2 followed by solvent extraction.
The recovery of palladium from aqueous acidic solutions in the past is also described in U.S. Pat. Nos. 4,331,634 and 4,578,250. In each of these patents, a process is described in which an aqueous acidic solution, originally acidic or one which may be obtained by adjusting the pH to render it acidic, is contacted with an organic phase comprised of an organic solvent containing an hydroxyoxime or etheroxime extractant. The organic phase is substantially immiscible with the aqueous acidic solution. After all, or a part, of the palladium is extracted from the aqueous acidic solution to the organic phase, the aqueous and organic phases are separated due to their immiscibility. The palladium now present in the organic phase in association with the oxime is then stripped from the organic phase by a second aqueous solution.
The stripping solution employed in the U.S. Pat. No. 4,331,634 is a strongly acidic solution, such as solutions of sulfuric, hydrochloric, perchloric or nitric acids. The organic phase in this patent may also contain an anionic phase transfer material or catalyst to aid in the extraction of the palladium from the original acidic solution. In the U.S. Pat. No. 4,331,634, the oximes employed as extractants for the palladium are hydroxyoximes.
In the U.S. Pat. No. 4,578,250, the stripping solution is an aqueous ammonia solution. The U.S. Pat. No. 4,578,250 utilizes oxime extractants, similar to those of the U.S. Pat. No. 4,331,634 patent, but which are however ether oximes in which the hydroxy group of the oximes of the U.S. Pat. No. 4,331,634 are converted to ether groups. In the U.S. Pat. No. 4,578,250, p-substituted phenol is employed to ensure ortho-acylation. The patent states "p-acylation yields material which does not selectively form a complex with the divalent palladium, and which is generally less stable under use conditions." No phase transfer materials are employed in this patent.
Another process for recovering palladium by extraction using an aldoxime, RHC.dbd.NOH, can be seen in Japanese Patent Application 61-238,927, laid open Oct. 24, 1986. Similarly, in "Solvent Extraction of Palladium (II) with Nonchelating Oximes with Diffferent Alkyl Chain Lengths", Yoshinari Baba, Katsutashi Inoue, Kazuharu Yoshizuka and Takashi Furusawa, Ind. Eng. Chem. Res., 29, 2111-8, 1990, there is described the use of non-chelating oximes such as dodecanal oxime, decanal oxime, octanal oxime and hexanal oxime. In Japanese Patent Applications 63-014,825 and 63-020,423, laid open on Jan. 22, 1988 and Jan. 28, 1988 respectively, the use of aliphatic oximes, such as 2-ethylhexanal oxime, as an accelerator is described in extractions employing phosphate or sulfidic extraction reagents.